Propellar having optimum efficiency in forward and rewarded navigation

ABSTRACT

A propeller in particular for a ship has blades each of which is pivotally arranged in the hub of the propeller so that the blade is capable of pivoting to and fro in an axial plane between forward and rearward positions. The blades are constructed such that each of the blade profiles formed as the intersecting face between a cylinder face coaxial with a propeller and a blade, is symmetrical in a position between forward and rearward positions which are determined by fixed stops in the hub and/or the simultaneous actions of the centrifugal force and the hydrodynamic pressure on the blade at a predetermined speed of rotation. This imparts a high efficiency and a quiet and steady operation to the propeller, when the ship navigates forwardly and rearwardly.

BACKGROUND OF THE INVENTION

1. FIELD OF THE INVENTION

The invention concerns a propeller in particular for a ship and havingblades each of which is pivotally arranged in the hub of the propellerso that the blade is capable of pivoting to and fro in an axial planebetween forward and rearward positions.

2. DESCRIPTION OF THE PRIOR ART

The energy which a propeller is capable of releasing when it is topropel a ship forwardly in the water depends upon the configuration ofthe propeller, in particular diameter, area, pitch and number of blades,in addition to the relative velocity of flow of the water and the actualnumber of revolutions of the propeller. The ratio of the energy releasedto the energy received by the propeller from the drive engine is calledthe efficiency of the propeller which is thus an expression of theability of the propeller to utilize the added energy. When the ship ispropelled by power the fuel consumption should be restricted to what isstrictly necessary for economic and environmental reasons. Therefore, itwill usually be attempted to design a propeller for a specific purposesuch that it has an efficiency as high as possible under the givenconditions of operation. One of the most important parameters includedin the calculations in this context is the shape of the blade profilesformed as intersecting faces between the blades and cylinder faces whichextend coaxially with the propeller. It is known that the intended goodefficiency can be obtained by providing these blade profiles with acurved central line between their edges and having the convex side ofthis line facing the same way as the direction of navigation.

The typical direction of navigation will usually be forward, which istherefore generally chosen as the way which the convex side of theprofile central lines is to face. Forward navigation can therefore takeplace with a good efficiency, which, on the other hand, is immediatelytransformed to an extremely poor efficiency when the ship is to bepropelled rearwardly, because the curvature of the profile then facesthe wrong way. In propellers having rigid blades the curvature and itsdirection are given once and for all when the propeller wasmanufactured, while the curvatures in propellers having pivotable bladesare changed in response to the position of these.

U.S. Patent specification 3,981,613 discloses a folding propeller whichis constructed particularly for improving the efficiency during rearwardnavigation. However, this is achieved merely by permitting the blades topivot to positions which are located on both sides of a plane whichcontains the axis of rotation of the blades and is at right angles tothe propeller axis.

It is common to the above-mentioned conventional propellers that theiraverage efficiency in forward as well as rearward navigation is notsatisfactory, and that the disadvantageous shape of the blades duringrearward navigation is moreover the cause of strong propeller noise andvibrations which spread into the ship to the inconvenience of those onboard.

Accordingly, there is a pronounced need for a new propeller structurewhose total efficiency is optimum in forward and rearward navigation,and which, also in the latter situation, has blade profiles imparting aquiet and steady operation to the propeller.

SUMMARY OF THE INVENTION

The novel and unique features of the invention, by means of which thisis achieved, are that the blades in a propeller having pivotal bladesare constructed such that, in an area at least extending between theinnermost and outermost end portions of the blade, each of the bladeprofiles, formed as the intersecting face between a cylinder facecoaxial with the propeller and a blade, is substantially symmetricalabout a straight line extending between the edges of the profile in aposition between forward and rearward positions which are determined byfixed stops in the hub and/or by the simultaneous action of thecentrifugal force and the hydrodynamic pressure on the blade at apredetermined speed of rotation. This changes the shape of the bladeprofile such that their curvature also has the convex side facing thesame way as the direction of navigation during reverse movement.

In a particularly advantageous embodiment to ensure optimum efficiencythe angular spacing of the position of symmetry from the propeller axisis determined by the fact that the cotangent to the angular spacing ofthe position of symmetry less the cotangent to the angular spacing ofthe rearward position divided by the cotangent to the angular spacing ofthe position of symmetry less the cotangent to the angular spacing ofthe forward position must numerically be of the same size as theproportion between the number of the operating hours of rearward andforward, respectively, navigation over a representative period.

The angular spacing of the position of symmetry from the propeller axismay advantageously be 90° when the propeller is used to the same extentin both directions, as is e.g. the case for bow propellers.

In many cases the blade profiles might be substantially symmetricalabout a central line transversely to the profile. Then, the edges of theprofile will usually be relatively sharp so that the blade cuts throughthe water in the same manner when the ship sails rearwardly as when itsails forwardly.

Alternatively, the blades may have asymmetrical blade profiles with awing shape which, when the ship sails forwardly, has a relatively roundleading edge and a relatively sharp trailing edge. This embodiment maybe chosen advantageously when special importance is attached to theeffect of the propeller during forward navigation, without this being atthe expense of the good efficiency in rearward navigation.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be explained more fully by the following descriptionof embodiments, which just serve as examples, with reference to thedrawing, in which

FIG. 1 schematically shows a propeller having a blade intersected by acylinder face to provide a blade profile,

FIG. 2 schematically shows a propeller having a pivotable blade in threepositions,

FIGS. 3A, B and C show a first embodiment of a blade profile in thethree positions shown in FIG. 2,

FIGS. 4A, B and C show a second embodiment of a blade profile in thethree positions shown in FIG. 2,

FIGS. 5A, B and C show a third embodiment of a blade profile in thethree positions shown in FIG. 2, but in a propeller structure in whichthe blades are rotated about their own axis while pivoting to and fro,

FIG. 6 is a lateral, partially sectional view of a folding propellerhaving two blades,

FIG. 7 is a lateral view of a bow propeller, and

FIG. 8 is an end view of the same.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The water surrounding a propeller in operation may in practice beconsidered to be incompressible. The water will therfore flow past onboth sides of the blade along intersecting curves between the blade andcylinder faces having the same axis as the propeller. This phenomenon isindicated in FIG. 1, in which the cylinder face is designated by thereference numeral 1, the propeller blade by the reference numeral 2 andthe intersecting curves between the blade and the cylinder face by thereference numerals 3a and 3b. The blade is secured to a schematicallyshown hub 4, which usually mounts at least two blades. As will be seen,the intersecting curves 3a, 3b define a blade profile that may beregarded as the active profile of the propeller, and which, in apropeller having pivotable blades, will change its shape in response tothe position assumed by the blade at a given moment.

When the water flows to the propeller during its rotation in thedirection shown by the arrow in FIG. 1, a negative pressure is createdon the front side of the propeller and a positive pressure on its rearside. The axial component of the resulting pressure constitutes thedrive pressure of the blade, so that the drive pressure of the propellerwill be the drive pressure of the individual blade multiplied by thenumber of blades which the propeller in question has. Usually, it isattempted to design a propeller blade such that with given dimensions itobtains the greatest possible drive pressure with the least possiblewater resistance. These factors greatly depend on the inclined positionof the profile chord with respect to the relative velocity of flow ofthe water, i.e. upon the pitch, as well as the shape of the activeprofile. If this is curved, a strong negative pressure will be createdon the more curved, or convex side and a smaller positive pressure onthe other side, as is generally known. When constructing the blades withactive profiles which are curved, a given propeller can therefore havean advantageously big drive pressure in the direction which thecurvature of the propeller points.

FIG. 2 shows a folding propeller of the type which is described in U.S.Patrnt specification 3,981,613. This propeller type is unique in thatthe blades can pivot from a folded position to a position having agreater angular distance from the propeller axis than 90°. Thisstructure entails that the propeller can have a relatively goodefficiency also in rearward navigation.

During a representative period, e.g. a calendar year, a ship willnavigate rearwardly for a specific number of hours h₁ and navigateforwardly for another number of hours h₂. Navigation in both directionscan take place at different propeller speeds either by changing thenumber of revolutions of the engine or by gearing the engine. However,in practice the speeds of rotation will be grouped around typical speedsor predominant speeds which the crew of the ship find expedient. Thesepredominant speeds of rotation therefore form the basis for thesubsequent considerations.

In the folding propeller shown in FIG. 2 the angular position of theblade with the propeller axis at the predominant reverse speed isindicated by α and the predominant forward speed by γ. A further bladeposition is shown between these two positions in which the blade has anangular distance β from the propeller axis. A representative activeblade profile in a first embodiment is shown in FIGS. 3A, B and C in thethree positions shown in FIG. 2. The profile has a central line Xextending from edge to edge and a transverse central line Y which isperpendicular to the central line X. In FIG. 3B the active profile issymmetrical about the central line X. This position is therefore calledthe position of symmetry of the blade. When the blade pivots from thisto the reverse position shown in FIG. 3A, the profile will curve withthe convex side pointing the same way as the ship navigates rearwardly.If, on the other hand, the blade is pivoted to the predominant forwardposition shown in FIG. 3C, the blade will curve in the oppositedirection, viz. with the convex side now pointing the same way as theship sails forwardly. No matter whether the ship thus sails forwardly orrearwardly, the active profile will therefore have an advantageouscurved shape which, in both cases, has the convex side in the directionin which the ship sails at the time in question. Contrary toconventional propellers, the propeller of the invention has a goodefficiency also when the ship navigates rearwardly, and blade profilescausing the propellers to operate quietly and steadily.

Utilizing the geometry prevailing when the blades of the propeller inthe various pivoted positions are intersected by coaxial cylinder faces,an optimum efficiency can be obtained when choosing for the position ofsymmetry an angular distance β which satisfies the equation ##EQU1## inwhich the designations are as stated above. This entails not only thatthe active profiles of the blades will be curved in the right way inboth directions of navigations, but also that the time of the navigationwill be included for determining the position of symmetry in a balancedmanner at such an angular distance that the overall efficiency throughe.g. a calendar year will be as great as possible.

In FIGS. 3A, B and C the active profile is also symmetrical about thetransverse central line Y, and it has relatively sharp edges at bothends. The blades will therefore act in the same manner, when they cutthrough the water, no matter whether the ship sails forwardly orrearwardly. This is not quite the case in the second embodiment shown inFIGS. 4A, B and C, which is asymmetrical about the central transverseline Y. In the position of symmetry shown in FIG. 4B the active profileis almost drop-shaped having a sharp edge in the rearward direction andrelatively round edge in the forward direction. This provides extremelygood flow conditions around the blades when the ship sails forwardly.However, as shown in FIG. 4A, the active profile curves advantageouslyin this case too in the direction of navigation when the ship navigatesrearwardly.

In the third embodiment shown in FIGS. 5A, B and C the propeller rotatesthe same way no matter whether the ship navigates forwardly orrearwardly, since the blades merely rotate about their own axis when thedirection of navigation is to be changed. Therefore, the active profilecan have an advantageous drop shape also for rearward navigation.

FIG. 6 shows a typical folding propeller which is generally designated5. The propeller has a hub 6 which is mounted on the shaft 7 of a ship,of which only a fragment of the outermost end is visible. Two pivot pins8 are arranged in the hub, and each of these pivotally mounts a blade 9.Each of these blades has a tooth segment 10 at the innermost end whichengages the tooth segment 10 of the opposite blade to synchronize thepivotal movements of the blades. In the case shown, the hub 4 moreoverhas a fixed stop in the form of an abutment 11 against which the bladerests when it is present in the prevailing forward position. Theprevailing rearward position is the position in which the blade is inequilibrium owing to the simultaneous action of the centrifugal forceand the hydrodynamic pressure when the propeller rotates at theprevailing speed of rotation. It is noted that the embodiment of thefolding propeller shown in FIGS. 4 and 6 just serves as an example. Theprevailing positions may either be the positions in which the blades arein equilibrium owing to the simultaneous actions of the centrifugalforce and the hydrodynamic pressure, or the positions may merely bedetermined by means of fixed stops in the hub, or be a combination ofthe two methods.

In many cases it is not desirable that the propeller is capable of beingfolded together completely. In that case, the blades can merely pivotthrough a certain angle about 90° between the two fixed stops in thehub. This is e.g. the case with the bow propeller 12 shown in FIGS. 7and 8, which operates in a tunnel 13 provided transversely through theship 14. The blades 15 of the bow propeller just need to be able topivot through a minor angle about a position in which the angulardistance from the propeller axis is 90° and the blade profiles aresymmetrical about a straight longitudinal central line. The blades cantherefore advantageously be constructed with active profiles which arecurved to exactly the same extent in the operating directions of the bowpropeller 12.

The invention is described above and shown in the drawing on theassumption that the propeller is to be used for a ship. However, it iscontemplated that the structure of the invention can also be used formany other purposes within the scope of the invention, e.g. axialventilators or axial turbines and stirring equipment.

We claim:
 1. A propeller for a ship, said propeller having a hub, bladesmounted on the hub of the propeller, pivot means connecting the bladesto the hub to enable the blades to pivot to and fro in an axial planebetween forward and rearward positions characterized in that the bladesare designed such that, in an area extending at least between innermostand outermost end parts of the blade, each of the blade profiles, formedas the intersecting face between a cylinder face coaxial with thepropeller and a blade, being substantially symmetrical about a straightline extending between edges of the profile in a position between saidforward and rearward positions, each of the blade profiles, formed asthe intersecting face between a cylinder face coaxial with the propellerand a blade, having a convex surface facing the direction of movement ofthe ship when the blades are pivoted to forward and rearward positionson opposite sides of the position of the blades when the blade profileis substantially symmetrical.
 2. A propeller according to claim 1,characterized in that each blade profile is substantially symmetricalabout a straight line between the edges of the profile when the bladehas an angular distance β with respect to the propeller axis satisfyingthe equation ##EQU2## in which α and γ are the angular distance of theblade from the propeller axis at rearward and forward, respectively,navigation when the blade is in a position of equilibrium and h₁ and h₂are the number of operating hours of rearward and forward, respectively,navigation over a cyclically recurring period.
 3. A propeller accordingto claim 1, characterized in that each blade profile is substantiallysymmetrical about a straight line between edges of the profile at anangular position of 90°.
 4. A propeller according to claim 1,characterized in that each blade profile is substantially symmetricalabout a central line transversely to the profile.
 5. A propelleraccording to claim 1, characterized in that each blade profile issymmetrical about a central line transversely to the profile.
 6. Apropeller according to claim 5, characterized in that each blade profilehas a relatively sharp edge in the rearward direction and a relativelyround edge in the forward direction.
 7. The propeller according to claim1 wherein said forward and rearward positions of said propeller bladesare determined by stops on the hub.
 8. The propeller according to claim1 wherein said forward and rearward positions of the propeller bladesare positions of equilibrium caused by simultaneous actions ofcentrifugal force and hydrodynamic pressure at a predetermined speed ofrotation of the propeller.